Parse shorthand multi-pronoun inputs (such as 'she/they')

[reeseovine]

Some requirements:

• It must not break existing functionality.
• If there is a conflict with a single pronoun set, the single set must take precedence.
• Tests and documentation should be updated.
• Write code in a separate branch and merge when it's ready.

[reeseovine]

Implemented in https://github.com/katacarbix/pronouns.js/commit/402764b206b405408d4b142ec882261211b23803